Entries Tagged as 'IGP'

RIPv2 Timers

There are four type of timers in Cisco implementation, three of them is coming from RFC2453, one of them is Cisco standard.

  • Update
  • Invalid (Gabage Collection Timer)
  • Flush
  • Hold Down (Cisco Standard)

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  1. Update Timer

    2. The update timer specifies the frequency of the periodic broadcasts. By default, the update timer is set to 30 seconds. Each route has a timeout value associated with it. The timeout gets reset every time the router receives a routing update containing the route.

  2. Invalid Timer

    3. When the timeout value expires, the route is marked as unreachable because it is marked invalid. The router marks the route invalid by setting the metric to 16. The route is retained in the routing table. By default, the invalid timer is 180 seconds, or six updates periods (30 x 6 = 180).

  3. Flush Timer

    4. A route entry marked as invalid is retained in the routing table until the flush timer expires. By default, the flush timer is 240 seconds, which is 60 seconds longer than the invalid timer.

  4. Hold-down Timer

    5. Cisco implements an additional timer for RIP, the holddown timer. The holddown timer stabilizes routes by setting an allowed time for which routing information regarding different paths is suppressed. After the metric for a route entry changes, the router accepts no updates for the route until the holddown timer expires. By default, the holddown timer is 180 seconds.

Tags: IGP, Networks by hengdu
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EIGRP Stub Routing

The detailed EIGRP Stub Routing is introduced in here. I couldn’t describe this better than that. :)

However, I do have my thinking for this feature and will present in here.

EIGRP Stub routing is basic idea to slove EIGRP stuck in active problem. It’s easy to illustrate in hub-and-spoke network that hub connects to WAN and all spokes have to go pass through hub to the outside world. So, the hub router doesn’t need to send QUERY to all spoke when WAN network changed. To enable “eigrp stub” on EIGRP process, hub router will not send QUERY to spokes.

We can do a simple testing for this feature. As we can see from diagram, R2 is a distribution router as hub and R3 is a access router as a spoke. R1 will be some router outside.

R1 <——–VLAN432—————-> R2 (Hub)<—————VLAN450——————-> R3 (Spoke)

When eigrp stub disabled on R3, we can do “debug eigrp package query“. We can see that R2 send QUERY to both VLAN432 and VLAN450.

23:02:06: EIGRP: Received QUERY on Vlan432 nbr 10.2.106.81
23:02:06:   AS 10, Flags 0×0, Seq 21/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely0
23:02:06: EIGRP: Enqueueing QUERY on Vlan450 iidbQ un/rely 0/1 serno 33-48
23:02:06: EIGRP: Enqueueing QUERY on Vlan432 iidbQ un/rely 0/1 serno 33-48
23:02:06: EIGRP: Sending QUERY on Vlan450
23:02:06:   AS 10, Flags 0×0, Seq 33/0 idbQ 0/0 iidbQ un/rely 0/0 serno 33-48
23:02:06: EIGRP: Sending QUERY on Vlan432
23:02:06:   AS 10, Flags 0×0, Seq 34/0 idbQ 0/0 iidbQ un/rely 0/0 serno 33-48

When eigrp stub is enabled, we can see that R2 will NOT send QUERY to vlan450 anymore. But it still send QUERY to VLAN432.

23:03:07: EIGRP: Received QUERY on Vlan432 nbr 10.2.106.81
23:03:07:   AS 10, Flags 0×0, Seq 24/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely0
23:03:07: EIGRP: Enqueueing QUERY on Vlan450 iidbQ un/rely 0/1 serno 81-96
23:03:07: EIGRP: Enqueueing QUERY on Vlan432 iidbQ un/rely 0/1 serno 81-96
23:03:07: EIGRP: Sending QUERY on Vlan432
23:03:07:   AS 10, Flags 0×0, Seq 40/0 idbQ 0/0 iidbQ un/rely 0/0 serno 81-96
23:03:10: EIGRP: Received QUERY on Vlan450 nbr 192.168.50.92
23:03:10:   AS 10, Flags 0×0, Seq 20/0 idbQ 0/0 iidbQ un/rely 0/0 peerQ un/rely0

Tags: IGP, Networks by hengdu
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OSPF Summarization

Briefly outline in here that OSPF has two ways to do summarization

For Internal Area: area <#> range <IP> <Mask>

The summarization has to be done on ABR. It actually said that, for area <#>, we are going to summary as this IP with this Mask and advertise it out by default. The summarization will be advertised as LSA Type 3 to the whole areas. The summarization will not suppress sub network, which is different with summary-address in OSPF and ip summary-address in RIP and EIGRP.

For External Area: summary-address <IP> <Mask>

The summarization has to be done on ASBR. (This command will do nothing if configured on ABR). It actually said that, all external route can be summrized as <IP> with <Mask>. It will be advertised to all OSPF area as LSA type 5. The advertising router is ASBR for the whole area, and most important, it will advertise back to IGP like RIP or EIGRP when doing mutual redistribution, since it will generate a discard route in the local routing table – “O    10.10.0.0/21 is a summary, 18:32:09, Null0″. The redistribution from OSPF to RIP, as an example, will do two steps.

  1. show ip route ospf to see all OSPF generated route.
  2. list all connected interface that advertised by OSPF

Therefore, we need route-map when advertise OSPF to RIP or EIGRP to filter out this summary route.

Tags: IGP, Networks by hengdu
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