Wednesday 24 March 2010

BSCI - OSPF - Default Route propagation

By default OSPF does not propagate a default route, and so you need to manually tell OSPF to distribute one from your Autonomous System Border Router (ASBR).

Depending on the type of Area you have employed in your network the way you propagate a default to all your routers will differ slightly.

In Normal Areas (OSPF areas all connected to Area 0) you can do the following:
1) On your ASBR apply a default route:
    R1(config)#ip route
2) Inject the default route in to OSPF:
    R1(config)#router ospf 1
    R1(config-router)#default-information originate [always]

- The [always] option allows you to advertise a default route from the ASBR even when one doesn't actually exist. This can potentially result in better stability for your network. For example, if a default route is learned from a different routing protocol such as RIP and this route for what ever reason starts to flap then every time the route changes type 5 LSA's will be sent into the OSPF domain from the ASBR.
- The [always] option helps prevent actions outside of the OSPF domain from affecting the routers/routes within the OSPF domain.

For Stub and Totally Stub areas the situation is different.

On an ABR you configure an area to be a stub. This in turn prevents type 5 LSA's (external route information) from being sent in to the stub area and in return a default summary route is propagated.

In a Totally Stubby area, this goes a step further. By configuring an area as a Totally Stubby area on the ABR you prevent Type 5 LSA's (for external routes) plus Type 4 and Type 3 LSA's (for inter-area summary routes) from being propogated. A default summary route replaces these types of routes.

In both cases, as a default route is automatically generated at the ABR, you do not require the default-information originate command.

Finally you have Not-So-Stubby-Area's (NSSA)
There are 2 ways to advertise a default route. NSSA ABR can generate a default route with or without a default route in its own routing table.
1) On the ABR connecting Area 0 to the NSSA area you force the ABR to generate a default route:
    R3(config)#router ospf 1
    R3(config-router)#area 8 nssa default-information originate

-With this example the ABR generates Type 7 LSA's with a link state ID of this is then advertised within the NSSA area

- NOTE -  NSSA ASBR can generate a default only when it has a default route in its routing table
The default route via the ASBR must be known through non-OSPF protocol

2) You can also use the 'no-summary' option when defining your NSSA area and create 'NSSA Totally Stub area':
    R3(config)#router ospf 1
    R3(config-router)#area 8 nssa no-summary

-In this example you are replacing the Type 3,4,(inter-area summary routes) and Type 5 LSA's(external summary routes) with a default summary route.This is just as you do for a Totally Stubby area.

Tuesday 9 March 2010

EIGRP Summarisation and NULL0

EIGRP Summarisation allows you to stream line the routing table making it more efficient. Fewer Routes listed results in less EIGRP updates being sent out and there fore less resources are consumed (Bandwidth, CPU ultilisation, load etc).

By default EIGRP performs auto-summarisation, that is to say that EIGRP will automatically summarise at a major class boundary during redistribution from EIGRP into a classful routing protocol (eg RIP).

EIGRP will also summarise at the major classful boundary when a route is advertised out of an interface that is on a different major class boundary.

In order to prevent a routing loop when summarisation is in effect (whether manual or automatic) a summary is automatically assigned to the NULL0 interface to prevent routing loops. If the router with a summary route received a packet for an unknown subnet that is part of a summarised range then the longest match ends up being the summary route itself (not a subnet of it) and so this is forwarded to the NULL0 interface and is dropped.

The idea is that the router is then prevented from forwarding the packet on to a default route and potentially creating a loop.

Benefits of summarising in this way include a smaller routing table leading to faster look ups, more specific routes will be hidden so if that specific route goes down the whole network does not need to recalculate the DUAL alogrithm, routing updates will be smaller and so limit the number of EGIRP Queries.

The down side of auto summarisation is that it won't factor in discontiguous networks. As a result you could have networks behind the same network advertised out of 2 different interfaces to 2 different networks resulting in 50% of traffic arriving in the wrong place.

To address this issue you can disable automatic summarisation and use manual summarisation.

Disabling automatic summarisation is as simple as this:
R1(config)#router eigrp 1
R1(config-router)#no auto-summary

Now the router will not perform summarisation and all available subnets will be advertised.

Manual summarisation is configured on a per-interface basis, when a summary route is applied a NULL0 summary route entry is immediately created in the routing table to help prevent loops.

To configure a manual summary route you do the following:
R1(config)#router eigrp 1
R1(config-router)#no auto-summary

R1(config)#int s0/0/0
R1(config-if)#ip summary-address eigrp [AS] [IP] [Subnet Mask]

e.g) R1(config)#int s0/0/0
R1(config-if)#ip summary-address eigrp 1

With a manual summary route, the summary route is advertised only if a more specific entry of the summary is present in the routing table. Otherwise it won't appear in the routing table.

CCNP ROUTE Cert Kit Giveaway

Rofi Neron over at has a Cisco CCNP Route Cert Kit to giveaway.

To enter check out his competition here:

Well worth the time and an excellent opportunity to bag some Cisco Press material.

Good Luck!!

Wednesday 3 March 2010

CCNP and Networking Academy students

Since Cisco announced their plans to move to the new 3 exam path back in January there's been much discussion at my local Networking Academy about what that actually means for those who have already passed some of the exams but won't finish the Academy programme until after July31st.

Cisco have posted the following information;
on the last page it says
Q. Until when will Networking Academy students be able to take the current CCNP certification exams?
A. The four current CCNP certification exams will be available to the general public through July 31, 2010 and to Networking Academy students using a special voucher through July 31, 2011. More information about how to obtain a special, non-discount voucher to enable students to continue to take the retiring exams will be communicated as soon as it is available. 

CCNP Exam Combinations:
BSCI+BCMSN+ISCW+ONT      = Last day is July 31st 2011
COMP+ISCW+ONT                  = Last day is July 31st 2011
BSCI+SWITCH+ISCW+ONT     = Last day is July 31st 2011
ROUTE+BCMSN+ISCW+ONT  = Last day is July 31st 2011 
ROUTE+SWITCH+ISCW+ONT = Last day is July 31st 2011 
BSCI+BCMSN+TSHOOT         = Ongoing
COMP+TSHOOT                    = Ongoing
BSCI+TSHOOT+SWITCH        = Ongoing

This should be a weight off anyone's mind who is doing their CCNP through a Networking Academy (including me!)