Term
| How do IS-IS routers know how to route out of their respected area? |
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Definition
| An L1/L2 router automatically advertises to all L1 routers (within its area) that it is a potential exit point of the area. L2 routers will default to the nearest attached L1/L2 router. |
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Term
| What is the relationship between the area address and the system ID when it comes to routing? |
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Definition
| The area address is used to route between areas, and the system ID is not considered. The system ID is used to route within an area, and the area address is not considered. |
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Term
| Why is it important to know the traffic patterns within your network and tune IS-IS for optimal path selection? |
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Definition
| Each router makes its own best-path decisions at every hop along the way, which means that there is a significant chance that paths will not be reciprocal. That is, return traffic can take a different path than the outgoing traffic. |
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Term
| What is route leaking in regards to IS-IS? |
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Definition
| Route leaking allows L2 routes into L1 routers in a controlled manner (like redistribution). To implement route leaking, and up/down bit in the TLV is used to indicate whether or not the route identified in the TLV has been leaked. If the up/down bit is set to 0 the route was originated within that L1 area. If the up/down bit is set to 1 the route has been redistributed into the area from L2. |
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Term
| What type of encapsulation is done with regards to an IS-IS PDU? |
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Definition
| IS-IS PDUs are encapsulated directly into an OSI data-link frame. There is no CLNP or IP header. |
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Term
| What are the four different types of PDUs defined by IS-IS? |
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Definition
- Hello PDU (ESH, ISH, IIH) - Used to establish and maintain adjacencies. - LSP - Used to distribute link-state information - Partial sequence number PDU (PSNP) - Used to acknowledge and request missing pieces of link-state information - Complete sequence number PDU (CSNP) - Used to describe the complete list of LSPs in a router's LSDB |
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Term
| What is the default lifetime for an LSP PDU? |
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Definition
| 1200 seconds is the default start value. |
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Term
| What are the 2 network types that are supported by IS-IS? |
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Definition
| Point-to-point networks and broadcast networks. |
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Term
| How does IS-IS work in NBMA networks? |
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Definition
| IS-IS has no specific support for NBMA networks. When creating static maps to map the remote IP address to the local DLCI on a Frame Relay interface, you should use the broadcast keyword. |
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Term
| What command is used to allow CLNS broadcasts and multicasts on a Frame Relay network? |
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Definition
| frame-relay map clns dlci-number broadcast |
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Term
| How are adjacencies established between routers that are running both L1 and L2 processes? |
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Definition
| Separate IS-IS adjacencies are established for L1 and L2 processes. If 2 neighboring routers in the same area run both L1 and L2, they establish 2 adjacencies, one for each level. The router stores the L1 and L2 adjacencies in separate L1 and L2 adjacency tables. |
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Term
| What's the difference between how IS-IS and OSPF form adjacencies between routers? |
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Definition
| IS-IS routers on a LAN establish adjacencies with all the other routers on the LAN, unlike OSPF, where routers establish full adjacencies only with the DR and BDR. |
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Term
| What is the default priority for an interface running IS-IS? |
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Definition
| Cisco router interfaces have a default L1 and L2 priority of 64. |
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Term
| What command is used to change the default priority of an interface running IS-IS? |
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Definition
| isis priority value [level1 | level2] |
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Term
| What is the role of a DIS in an IS-IS broadcast link? |
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Definition
| A DIS is somewhat like a DR in a broadcast network. Adjancencies still form between all routers, but update are handled by the DIS. |
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Term
| How are L1 and L2 LSPs distributed within a network? |
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Definition
| The link-state information for the 2 levels is distributed separately, in L2 LSps and L2 LSPs, each of which may come from a different DIS. |
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Term
| What is the default hello interval between ISs? |
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Definition
| The default interval is every 10 seconds |
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Term
| What type of hello messages are used to establish and maintain neighbor adjacency between ISs? |
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Definition
| IIHs are used to establish and maintain neighbor adjacencies between ISs. |
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Term
| What is the multicast MAC address used for IIHs between L1 and L2 neighbors? |
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Definition
| L1 announcements are sent to the multicast address 0180.C200.0014. L2 announcements are sent to the multicast address 0180.C200.0015. |
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Term
| What is the default interval for the DIS in an IS-IS area? |
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Definition
| The default hello interval for the DIS is 3 times faster than the interval for other routers, so that DIS failures can be detected quickly |
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Term
| What is the default hold time for a router that is running IS-IS? |
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Definition
| The hold time is determined by the hello multiplier and the hello time. The default multiplier is 3, so that would be a hold time of 30 seconds if the hello time is default (10 seconds) |
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Term
| How are sequence number PDUs (SNPs) used to maintain LSDB synchonization between neighbors? |
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Definition
| SNPs are used to acknowledge the receipt of LSPs and to maintain LSDB synchonization. The 2 types of SNPs are CSNPs and PSNPs. The main difference is that CSNPs contain summaries of all LSPs in the LSDB, whereas PSNPs contain only a subset of LSP entries. |
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Term
| How are CSNP broadcasts handled in a broadcast network? |
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Definition
| In broadcast networks, only the DIS transmits CSNPs |
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Term
| What is a major difference between an IP address and an IS-IS NET address? |
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Definition
| A NET address identifies a device (an IS or ES), not an interface. |
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Term
| How do IS-IS routers calculate ES reachability? |
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Definition
| Routers calculate ES reachability with a partial route calculation (PRC), based on the L1 and L2 SPF trees. |
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Term
| What command enables IS-IS as an IP routing protocol and assigns an optional tag to the process? |
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Definition
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Term
| What command is used to assign a network entity title (NET) to a router? |
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Definition
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Term
| What command enables an interface to use IS-IS to distribute its routing information? |
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Definition
| ip router isis [area-tag] |
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