Subnets between core and access/distribution layers & routing between layers

Discussion in 'Cisco' started by J, Dec 13, 2006.

  1. J

    J Guest

    I'm seeking out advice on connecting multihomed access & distribution
    devices back to a redundant core. Currently I'm working with a mix of

    1) A /28 or /27 VLAN was created on each core router to contain the
    aggregate links to the lower layered devices. The access/distribution
    devices have a L3 interface in this subnet.

    2) A /30 is used between the core routers and the access/distribution
    layer devices.

    3) A /29 with HSRP is used across the core routers on L3 VLAN
    interfaces and across the Ethernet interfaces on the
    access/distribution devices. Each core router has VLAN with a L3
    address in the /29 and a common standby IP also in the /29. The
    access/distribution layer device has 2 Ethernet links back to the core
    that are 1Q trunks. That VLAN has a L3 address in the /29. This only
    works on a few devices, namely switches used for aggregation or routers
    with L2 ports like ENET HWICs.

    I'm trying to figure out what the best approach is in this service
    provider environment. I personally favor #2. I believe this fits in
    better with a clean hierarchical IGP. I'm moving from OSPF to IS-IS in
    the very near future and would like to better utilize areas than they
    have been in the past (OSPF area 0 is touching every single device).
    #1 would make it difficult to separate IS-IS areas I think, especially
    if I use the VLANs for inter-connection VLANs for more than one type of
    device that should be in separate areas.

    IPs aren't a concern. We're migrating from a public to a private IP
    infrastructure for the heart of this service provider. This will make
    ARIN happy.

    On a related topic, I'm also soliciting advice on the basic
    configuration moving forward for edge and aggregation devices. Like I
    said earlier, we're moving to a pure IS-IS environment with a redundant
    core. All devices will be multihomed to the redundant core routers.
    All devices whether it be an access server, router terminating ATM PVCs
    for DSL, or a pair of aggregation switches that mulithome CMTSs will be
    running IS-IS and will be advertising the local routes back to the core
    (some with summarization like the access servers and ATM routers). The
    access/distribution layer devices are set up in one of 2 ways right
    now. Either their default route is originated in the IGP or they use a
    static default route (and in some cases still receive an default route
    from the IGP but don't use it). Should I originate a default route and
    remove the static default, should I use the static default route, or
    should I have a higher cost static route as a backup (rather useless I
    think)? Is there a best practice on this one that I can cite?

    I just thought of another quick question. Do many people use (or find
    useful) 1Q trunks in a service provider network? I only have 2 1Q
    trunks in this entire network. Personally I would rather use L3
    routing between capable devices wherever possible. I suppose if I use
    a pair of switches for aggregation (EMI code on 3750s or 4948s for
    example) I could use a pair of 1Q trunks between then for HSRP
    purposes. Do any other uses come to mind?

    J, Dec 13, 2006
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