Hello everyone,
Today I will share with you IS-IS aggregated route.
1. Route aggregation
If a route (direct, learned, or imported) is being advertised in an LSP and is to be aggregated, the route is first deleted from the LSP.
After route aggregation is complete, the aggregated route is advertised in the LSP of the specified level.
2. Longest match
If a route can be aggregated by two routes, IS-IS selects the aggregated routes with a long mask for aggregation according to the longest match principle. The following is an example:
|
Destination/mask |
Exit interface |
Next hop |
|
100.1.1.1/16 |
ethernet6/0/0 |
10.10.10.2 |
|
100.1.2.1/24 |
ethernet6/0/0 |
10.10.10.2 |
|
100.1.3.1/24 |
ethernet6/0/0 |
10.10.10.2 |
|
100.1.4.1/24 |
ethernet6/0/0 |
10.10.10.2 |
|
100.1.5.1/24 |
ethernet6/0/0 |
10.10.10.2 |
Table 1
In the preceding table, the five static routes can be aggregated by 100.1.0.0/16 or 100.0.0.0/8. However, if the two aggregated routes are configured at the same time, the five routes are aggregated by 100.1.0.0/16 according to the longest match rule, but not aggregated by 100.0.0.0/8. Therefore, only the route 100.1.0.0/16 is advertised in the LSP, and the route 100.0.0.0/8 is not advertised.
3. Aggregating direct routes
Direct route summarization is the simplest. The following describes the aggregation process in Level-1 and Level-2 scenarios.
Example 1: Configure a Level-1 aggregated route.
RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. The two routers are on the network segment 1.1.1.0/24. RTA is configured with the aggregated route 1.1.0.0/16 of level-1 (the aggregated route is advertised in the LSP of this level). The direct route 1.1.1.0/24 is aggregated and then deleted from the level-1 LSP. The aggregated route 1.1.0.0/16 is advertised through the level-1 LSP. RTB learns the aggregated route from the level-1 routing table and advertises the aggregated route through the level-2 LSP (automatic leaking).
Example 2: Configuring Level-2 Aggregated Routes
RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. The two routers are on the network segment 1.1.1.0/24. RTA is configured with the aggregated route 1.1.0.0/16 of level-2 (indicating that the aggregated route is advertised in the LSP of this level). 1.1.1.0/24 of the direct route is aggregated and deleted from the level-2 LSP. The aggregated route 1.1.0.0/16 is advertised in the level-2 LSP. RTB learns the aggregated route in the level-2 routing table. If RTB is configured with the route leaking from level-2 to level-1, the aggregated route is advertised through the Level-1 LSP of RTB.
4. Aggregating learned routes
Only the learned routes advertised in LSPs can be aggregated. The following two examples are used for description.
Example 1: Configure level-1 aggregated routes.
As shown in the preceding figure, RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. RTB imports five static routes in Level-2. The Level-2 routing table of RTA can learn five IS-IS routes from RTB. The Level-2 routing table of RTA can learn five IS-IS routes from RTB. Because IS-IS is configured to leak routes from Level-2 to Level-1, the five routes are advertised through the Level-1 LSP of RTA. In addition, RTA is configured with Level-1 aggregated routes, the five routes are aggregated into one route 100.1.0.0/16 and advertised in the Level-1 LSP of RTA.
Note: For learned routes, the level (level-1) specified in summary 100.1.0.0 255.255.0.0 level-1 indicates that the aggregated route is to be advertised in the LSP of this level. The actual aggregated route is the route in the routing table of the opposite level.
Example 2: Configure a Level-2 aggregated route.
As shown in the preceding figure, RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. RTB imports five static routes to the Level-1 routing table. The Level-1 routing table of RTA can learn five IS-IS routes from RTB. The five routes are advertised (automatically leaked) in the Level-2 LSP of RTA. Currently, RTA is configured with Level-2 aggregated routes (if the level is not specified when the summary is configured, by default, the five routes are aggregated into one route 100.1.0.0/16 and advertised in the Level-2 LSP of RTA.
5. Aggregating imported routes
If IS-IS is configured to import Level-1 routes, these routes are added to the Level-2 routing table, and vice versa. Therefore, if level-1 aggregated routes are configured, the level-2 routing table is searched. The following example describes how to aggregate imported routes.
Example 1: Configure level-1 aggregated routes.
RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. RTB imports five static routes to Level-1. RTB is configured with Level-1 aggregated routes. The five imported routes are aggregated and advertised in Level-1 LSPs. RTA learns the aggregated route from the Level-1 routing table and advertises the route in the Level-2 LSP. During route summarization, Level-2 routing tables are searched because imported routes are placed in the routing tables of opposite levels.
Example 2: Configure a Level-2 aggregated route.
RTA and RTB establish a Level-1-2 neighbor relationship through Ethernet interfaces. RTB imports five static routes to Level-2. RTB has been configured with Level-2 aggregated routes. The five imported routes are aggregated and advertised in Level-2 LSPs. RTA's level-2 routing table learns the aggregated route. If RTA is configured with level-2 to level-1 route leaking, the aggregated route is advertised through the level-1 LSP of RTA. During route summarization, the device searches the level-1 routing table for imported routes because the imported routes are placed in the routing table of the opposite level.
6. Refreshing LSPs
Route aggregation causes LSP content changes and LSP updates.
That is all I want to share with you! Thank you!
